1. Field of Invention
The present invention relates generally to the field of lithography. More specifically, the present invention relates to removal of debris from a lithographic apparatus.
2. Description of Related Art
In a lithographic apparatus, the size of features that can be imaged onto the substrate is limited by the wavelength of the projection radiation. To produce integrated circuits with a higher density of devices, and, hence, higher operating speeds, it is desirable to be able to image smaller features. While most current lithographic projection apparatus employ ultraviolet light generated by mercury lamps or excimer lasers, it has been proposed to use shorter wavelength radiation, e.g., about 13 nm. Such radiation is termed extreme ultraviolet (EUV) or soft x-ray, and possible sources include, for example, laser-produced plasma sources, discharge plasma sources, or synchrotron radiation from electron storage rings.
Some extreme ultraviolet sources, especially plasma sources, emit substantial amounts of contaminant molecules, ions, and other (fast) particles. If such particles are allowed to reach the illumination system, which is downstream of the radiation source, or further downstream in the apparatus, they can damage the delicate reflectors and/or other elements, and cause build-up of absorbing layers on the surfaces of optical elements. Such damage and built-up layers cause an undesirable loss of beam intensity, which necessitates increasing exposure times and, hence, reducing machine through-put. The built-up layers and damage can be difficult to remove or repair. To prevent contaminant particles from reaching the illumination system, it has been proposed to provide a physical barrier or window in the exit from the radiation system or the entrance to the illumination system. However, such a window is itself prone to damage from the contaminant particles and build-up of absorbing layers. Also, since most materials absorb extreme ultraviolet radiation at the wavelengths proposed for use in lithography, the window, even when new and clean, will absorb a significant proportion of the beam energy, thereby reducing throughput. This absorption can cause thermal stress in the window even leading to breakage of the window.
EP-A-0 957 402 discloses a contamination barrier which uses a hollow tube, positioned between the last solid surface of the projection system and the substrate, that is flushed with gas flowing towards the substrate to prevent contaminants emitted from the resist being deposited on the projection lens.
EP-A-1 223 468 describes a lithographic projection apparatus and device manufacturing method in which a contaminant barrier comprising an ionization device is introduced. Such an ionization device may, for example, be an electron source or a plasma generated by capacitive or inductive RF discharge or AC discharge. This is a relatively complicated solution to remove undesirable contaminants.